Finite State Machine Engine

Compile a validated versioned transition table and evaluate one pure guarded transition per event. Repository transactions atomically record new state, idempotency outcome and effect descriptors for later dispatch.

1. Validate declared states, events, target states and uniqueness of each source/event pair. Pin the definition version to each machine instance.
2. Under the repository transaction, check the event registry before expectedVersion so a successful retry can return its original result. Reject an event ID reused with different content.
3. Find the transition, evaluate its pure bounded guard against immutable state and compute the reducer result. Reject absent transitions or failed guards without publishing mutations.
4. Commit state version, event outcome and outbox intent together. Dispatch effects separately with destination idempotency; define upgrades and terminal states explicitly.

• An ambiguous transition table fails validation.
• Guard rejection leaves state and outbox unchanged.
• Concurrent events with one expected version cannot both advance state.
• An identical retried event returns its original result after later transitions.

Trade-offs to explain

• Pure guards cannot make fresh network-dependent decisions without modeled input events.
• A transactional outbox still requires duplicate-tolerant external effects.
• Pinning definition versions simplifies recovery but requires an explicit upgrade strategy.